Two-thirds of the planet's surface; depths not sounded till well into the 19th century; home to the blue whale, history's largest animal; poetic touchstone for that which is not easily crossed over: The ocean is vastness defined. And it was once thought, by everyone from Jules Verne to biologist Thomas Huxley, to be inexhaustible.

Alas, this wishful thinking has long since gone the way of free lunches, final frontiers and easy oil. Most of the fish stocks we rely on for seafood have been pushed to or beyond the limit of their ability to replenish themselves, and dilution, as the old saw had it, is no longer the solution to pollution -- from sewage to plastic debris to bubbling crude, the seas are full up.

The oceanic domain can still inspire hopes and fantasies, however, as it always has. Fittingly, Julia Whitty's "Deep Blue Home" is a dream of a book, vivid yet languorous, rich in detail, richer still in emotional impact. By anchoring her wide-ranging meditation to personal memories of a decades-distant season of ornithological research in the Gulf of California, Whitty distills the oceanic vastness into something bright, enticing and just manageable enough to be captured in a bottle. She also crystallizes the particular frustration of scientists who, while striving to understand the complex webs of marine life, have watched them torn asunder faster than they can be catalogued, let alone conserved.

In 1980, Whitty writes, when she first visited tiny Isla Rasa in the finger of inland sea that Steinbeck knew as the Sea of Cortez, 30,000 female leatherback turtles nested along Mexico's western shores. She recalls one "in the last pulse of light before darkness . . . form[ing] a perfect mirror-image twin with the surface: a two-headed turtle, jellyfish tentacles streaming from the corners of her mouths, like cellophane noodles in a silver broth." But then, as Whitty writes, "everything changed." By 1996, fewer than 900 leatherbacks remained anywhere in the Central and South American Pacific, the rest done in by pollution and choking garbage, indiscriminately lethal fishing gear, coastal development and the wholesale collection of eggs.

"Deep Blue Home" can be trancelike. Whitty, an accomplished documentary filmmaker, unspools scores of vignettes of life in remote biological research camps and on film shoots throughout the world's oceans, and intersperses these with allegorical references to Hindu mythology and some of the finest scientific descriptions of sea life and ocean dynamics that this former oceanographer has ever read. But the surprise is that an author who is still young can narrate from firsthand observation the passage of the ocean from vast and inexhaustible to something still wonderful but diminished, like a penned bison or an ailing King Lear.

The truth is, we all can tell this story -- our world has changed more in the past five decades than in the previous period of human history, and it has been changed at our hands. Nor has this been for the most part because of extraordinary events such as the ongoing Gulf of Mexico catastrophe, but instead as a result of a million mundane decisions we have all made, unknowingly or uncaringly, to treat the ocean -- the planet -- as inexhaustible when it is not. Global warming. Ocean acidification. Ecosystem changes including the extirpation of large predator fish and mass die-offs of tropical corals. In the course of a single human lifespan, we have altered the ocean in ways usually seen only over geological time.

As a diver and filmmaker, Whitty examines the changing ocean from within its embrace. Paul Greenberg probes the salty depth from above, hand firmly grasping the working end of a fishing rod. An angler who has written about fish for almost as long as he has pursued them, Greenberg focuses, intently, on what he rightly calls our last truly wild food source.

The story of overfishing has often been told, though seldom this well. Greenberg considers four iconic creatures -- salmon, bass, cod and tuna -- to lay bare the cycle of discovery, exploitation and collapse that has touched or threatens practically every important food fish in the seas. He seamlessly integrates the decline of wild fish with the rise of fish-farming, noting rightly that humanity is in the process of domesticating the oceans, as we long ago tamed the land, and that eliminating all but a few primary food species is a natural consequence. In writing clearly and engagingly about the place of fish in global food markets, he manages also to convey the often-missed reality that fish are not just food, or even animals, but wildlife.

"Four Fish" and "Deep Blue Home" are as different in pacing and approach as two such twinned books could be, but they share more than a watery muse. Refreshingly, they are reminders that science and nature writing can be accurate as well as engaging, accessible and true, that a strong narrative and strong science can coexist, and that both are buoyed when they do.

Together, Whitty and Greenberg tell a profound story of loss and point to a few sparks of hope peeking dawn-like across the storm-tossed horizon. In both books the sea itself, with its many wondrous forms of life, is the true and very sympathetic protagonist. The stories are grim, but the conclusion of this saga has not been written. As the chief agents of doom, we have a unique opportunity to help a happier ending emerge. If these two books, read in the context of the disastrous Deepwater Horizon oil spill, can't help us muster the collective will to do so, it's hard to imagine what could.